This invention relates to imaging devices and, more particularly, to solid state infrared (IR) imaging devices.
Heretofore, problems have been incurred in IR imagers because of the extremely high level of background radiation (relative to level of IR "signal" radiation) which is encountered in the IR region of the spectrum. This has made IR imaging difficult because the background radiation tends to mask the IR signal radiation and results in an output signal having a low signal-to-noise ratio.
Also, solid state IR imagers now known have a limited dynamic range due to the relatively short integration times now possible with prior art imagers. This condition results from the fact that the target array sensing sites reach their saturation level in a very short time due to the high levels of background radiation.
One approach to the resolution of these problems is represented by U.S. Pat. No. 3,840,740 issued on Oct. 8, 1974, to Stewart and assigned to the assignee of the invention herein. The approach taken here was to provide a solid state imager having a control circuit to separate background and signal charge within each sensing site so that the signal charge could be read out independently of the background charge. The performance of IR imagers constructed in accordance with this patent, while improved, still has a sensing site integration time which is primarily determined by the level of background radiation and the saturation level of the sensing site.
Another approach is represented by U.S. Pat. No. 3,806,729 issued on Apr. 23, 1974, to Caywood. The Caywood patent is directed to an imager which is capable of separating a composite charge generated by exposing the image to radiation from a scene into a signal-charge component and a background-charge component by alternatively exposing an array of CCD sensing sites to a scene and to a source of background radiation and coupling the composite scene-related charge and the background-related charge to a differential detector, thereby eliminating background-charge component from the differential detector output. This approach still, however, provides an imager which has an integration time and dynamic range limited by the level of background radiation and the saturation level of the CCD sensing site initially receiving the incident radiation.
Accordingly, it is an object of this invention to provide an IR imager having a improved dynanic range, and an integration time which are not limited by the level of background radiation.
These and other objects will be apparent from a consideration of the following detailed description and accompanying claims.